The global spread of Human Immunodeficiency Virus (HIV) infection in women of childbearing age has resulted in a substantial increase in mother-to-child transmission (MTCT) of HIV. In the absence of intervention, the rate of MTCT can reach 35%. Antiretroviral therapy (ART) has significantly reduced the incidence of MTCT; however, such treatment is not widely available in developing countries, and ART does not affect the initial transmission of the virus through the epithelium. To initiate systemic infection, HIV must first be transmitted across oropharyngeal and/or intestinal mucosal epithelia of neonates/infants and then infect susceptible immune cells, but the mechanisms of HIV transmission across neonatal/infant mucosal epithelia is poorly understood. We have developed two novel and complementary models for the study of epithelial transmission of HIV: monostratified polarized oral epithelial cells and polarized oriented oral and intestinal tissue explants of adult and fetal/infant origins. We have shown that cell-free and cell-associated HIV transmission may occur through the fetal/infant oral epithelium but not the adult oral epithelium. Our preliminary data show that HIV-associated phosphatidylserine (PS) interaction with the PS receptor T-cell immunoglobulin and mucin domain 1 (TIM-1) may facilitate cell-free viral transcytosis through the neonatal/infant oral mucosal epithelium. HIV-infected cells of the mother from cervical/vaginal fluid and breast milk may penetrate into neonatal mucosal epithelium by interaction of adhesion molecules of macrophages/lymphocytes with mucosal epithelia. We also found that the anti-HIV innate proteins HBD-2 and - 3 inactivate virus during its transcytosis in the vesicular compartment through indirect interaction with gp120. Investigation of the molecular mechanisms of HIV transmission via neonatal/infant oral and intestinal epithelia may lead to a better understanding of the pathogenesis of HIV MTCT and the development of a new preventive therapeutic strategy against HIV MTCT. Accordingly, the specific aims of this proposal are: 1. To study the role of HIV macropinocytosis and endocytosis in viral transmission across fetal/infant oral and intestinal epithelia. 2. To study the molecular mechanism of HIV-infected macrophage penetration into fetal/infant oral and intestinal epithelia. 3. To investigate the mechanisms of HBD-2- and -3-mediated intracellular inactivation of HIV in fetal/infant oral and intestinal epithelia. The results of these studies will provide new information on the molecular mechanisms of MTCT of HIV via fetal/neonatal oral and intestinal mucosal epithelia and will open new avenues for designing antiviral drugs that specifically block HIV MTCT through mucosal epithelia.